Method and arrangement for improved model order selection

ABSTRACT

In a method of enabling model order selection for joint channel synchronization and noise covariance estimation of at least one received signal in a wireless communication network, generating S 0  a spatially and temporally stacked signal model by stacking successive samples of temporally adjacent received signal vectors and corresponding training vectors, computing S 1  a noise variance matrix for each hypothesized synchronization position, channel length and stacking order, based on the stacked training symbols: determining S 2  a best synchronization position for the received signal, based on the stacked training vectors, by jointly determining the best synchronization position for the received signal and estimating a channel length and a stacking order for said signal model based on the stacked training vectors.

TECHNICAL FIELD

The present invention concerns wireless telecommunication systems ingeneral, and specifically methods and arrangements for improved modelorder selection for joint synchronization, channel estimation and noisecovariance estimation in such systems.

BACKGROUND

The growing popularity of mobile services has resulted inever-increasing interference levels caused by the closer proximity ofusers, and in the case of time division multiple access (TDMA) basedsystems by a tighter frequency reuse. As a result, mutual interferenceamong users occupying the same radio channel has become a major sourceof signal disturbance. The ability to suppress co-channel interferencehas become increasingly important for mobile receivers in cellularsystems with tight reuse. This has led to the development of severaltechniques for interference suppression in the receiver units of thebase transceiver stations (BTS) or mobile stations (MS).

Multi-branch diversity or array processing is a class of commonly usedtechniques for suppressing interference, in which multiple versions ofthe same transmitted signal are produced and processed jointly in thereceiver in order to cancel one or more interfering signals. Thedifferent signal versions may be obtained by using multiple receivingantennas, by sampling the received signal over the baud rate oftransmission (i.e. over sampling), by separating in-phase (I) andquadrature-phase

of the signal, or by combinations of these. The method of separatingin-phase and quadrature-phase of the signal is commonly referred to assingle-antenna-interference cancellation (SAIC) and has recentlyreceived much attention in the so called GERAN standardization.

In conventional array processing, the interference is typically modeledas temporally (across time) and/or spatially (across different signalversions) colored noise. By performing proper spatial and/or temporalnoise whitening, the interference can be suppressed substantially. Suchwhitening operation may be performed before or duringdemodulation/equalization.

In order to suppress the noise or interference through spatial-temporalwhitening, the receiver typically requires an estimate of a certainspectral property of the noise, such as the noise covariance matrix.From such spectral property, a whitening filter can then be derived towhiten, and therefore suppress, the noise. If the statistics ofinterference can be assumed to be approximately stationary over the databurst, the estimation of the noise spectral property may be performedover a sequence of training symbols in each data burst that is known tothe receiver.

In addition, the demodulator or equalizer of the receiver must also beable to synchronize to the beginning of a data burst in order to begindemodulation. The synchronization process is typically done jointly withchannel estimation over the training sequence. When spatial/temporalwhitening is performed on the received signal to suppress noise orinterference, the operating carrier-to-interference power ratio (C/I)can be changed so drastically that the ordinary method ofsynchronization and channel estimation, such as the least squares (LS)method, can no longer produce an accurate synchronization position. As aresult, the reliability of synchronization and channel estimationbecomes a bottleneck of the overall receiver performance.

One known way of improving synchronization and quality of channelestimation in a multi-branch receiver is to first perform a certaininitial synchronization and channel estimation, such as the LS channelestimation, and then estimate the noise covariance matrix or functionbased on the residual signal after channel estimation. From theestimated noise covariance matrix, a whitening filter can be computedusing the well-known Whittle-Wiggins-Robinson Algorithm (WWRA). Theproblem with this approach is that the initial synchronization andchannel estimation (before whitening) may not produce an accurateestimate of the synchronization position and the channel estimate. As aresult, the statistics of the residual signal obtained from the initialsynchronization and channel estimation may not be representative of thestatistics of the actual noise or interference.

To overcome this, one known technique [1] is the so called IndirectSpatio-Temporal Interference Rejection Combining (Indirect ST IRC),which is a joint synchronization, channel estimation and noisecovariance estimation technique. The use of this technique in thereceiver algorithms for BTS or MS results in substantial interferencesuppression.

The technique described in [1] gives a method to jointly estimate thesynchronization position, channel, and noise covariance matrix, given abaseband model for a received signal containing a known trainingsequence. However, the length of the channel and the dimension of thenoise covariance matrix are assumed to be known. The choice of thechannel length and the dimension of the noise covariance matrix will bereferred to as the model order selection problem in the followingdetailed description.

Existing solutions to the model order selection problem can be dividedinto two groups. In the first group, the order of the model is fixed,and can be guessed or deduced from field measurements and subsequentlyhard coded into the algorithms. In the second group there are the ad hocmethods based on simulations. In this methodology, a statisticalregression is used to produce a table. The regression is made fromsimulation-generated data.

Neither of these two groups is satisfactory. The main disadvantage ofchoosing a fixed model order is that it lacks the flexibility needed tocope with the diverse deployment scenarios found in mobile networks. Themain disadvantage of the ad hoc methods is that the mobile system may beput to work in environments that do not necessarily fit the simulationconditions or the test cases chosen by the system designers.

Therefore, there is a need for improved methods and arrangements formodel order selection to enable improved ST IRC.

SUMMARY

A general object of the present invention is to enable an improvedtelecommunication system.

A further object of the present invention is to provide an improvedmethod of model order selection.

Another object of the present invention is to enable joint determinationof a best synchronization position, channel length and model order for asignal model.

These and other objects are achieved by the attached set of claims.

According to a basic embodiment, the present invention comprisesgenerating S0 a spatially and temporally stacked signal model bystacking successive samples of temporally adjacent received signalvectors and corresponding training vectors, computing S1 a noisevariance matrix for each hypothesized synchronization position, channellength and stacking order, based on the stacked training symbols:determining S2 a best synchronization position for the received signal,based on the stacked training vectors, by jointly determining the bestsynchronization position for the received signal and estimating achannel length and a stacking order for said signal model based on thestacked training vectors.

An arrangement according to the invention enables the execution of themethod steps.

This is then utilized in subsequent known steps of computing a noisecovariance estimate, an estimate of the fictitious channel and anestimate of the whitened channel, to enable an improved interferencecancellation in a telecommunication system.

Advantages of the present invention include:

-   -   Improved model order selection    -   Improved joint whitening/synchronization    -   Low computational complexity    -   Easy implementation in digital signal processors or other        electronic equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 illustrates a schematic system in which the present invention isapplicable:

FIG. 2 is a schematic flow chart of an embodiment of a method accordingto the present invention:

FIG. 3 is another schematic flow chart of an embodiment of a methodaccording to the present invention in relation to prior art:

FIG. 4 is a diagram illustrating the relation between BLER and SNR in asensitivity limited environment and how embodiments of the presentinvention enables an intelligent choice of the best model for differentenvironmental conditions:

FIG. 5 is a diagram illustrating the relation between BLER and C/I forvarious models in an interference limited environment and how thepresent invention enables an intelligent choice of the best mode fordifferent environment conditions;

FIG. 6 illustrates an arrangement according to an embodiment of thepresent invention.

ABBREVIATIONS

BTS Base Transceiver Station

EDGE Enhanced Data Rates for Global Evolution

GSM Global System for Mobile Communications

MS Mobile Station

SAIC Single Antenna Interference Cancellation

ST IRC Spatio-Temporal Interference Rejection Combining

TDMA Time Division Multiple Access

DETAILED DESCRIPTION

The present invention will be described with reference to the attacheddrawings, and in the context of a GSM/EDGE-based telecommunicationsystem as illustrated by FIG. 1. However, it is equally applicable toany other TDMA-based telecommunication system

The system in FIG. 1, in a known manner, comprises a core network towhich one or more base station controllers (BSC) are associated via theinterface A/Gb. Each BSC is associated with one or more base transceiverstations (BTS) via an interface Abis, each such BTS is configured forcommunicating with one or more mobile stations (MS) over an interfaceUrn.

One of the criteria of the present invention is the nature of thestacked signal vector to be evaluated. This has been described in WO2006/136875[1]. In short, the patent document discloses system and amethod in a radio receiver for joint synchronization and noisecovariance estimation of a received signal. A spatially and temporallystacked signal model, whereby successive samples of temporally adjacentreceived signal vectors and corresponding training vectors are stacked,is used in the derivation of the estimation problem. The Toeplitzstructure of the channel response matrix is neglected in the formulationof the estimation problem. The resulting estimator jointly estimates asynchronization position, a channel response matrix, and a noisecovariance matrix. An estimate of a whitened channel is then computedbased on the noise covariance matrix and an estimate of the channelresponse matrix.

As stated previously, the above described method assumes that thechannel length and the stacking order or model order are knownparameters. The following description of the present invention aims todescribe an improved method that jointly provides an estimate of thebest synchronization position, the channel length, and the stackingorder.

The basic model order selection problem is the following. A data set andan identifiable mathematical model for the data are given. The goal isto obtain the best estimate of the model parameters. If the number ofparameters in the model is unknown, then the models with more parameterswill always yield a better fit to the known data set than the modelswith fewer parameters, regardless of the number of parameters in thetrue model. The order selection problem is to estimate the true numberof parameters.

The present invention provides a solution to the model order selectionproblem for the indirect ST IRC as described in [1], by giving a closedform mathematical expression for an estimate of the model or stackingorder. Specifically, a formula for the length of the channel and thedimension of the noise covariance matrix is given.

Model order selection problems arise frequently in many fields ofscience and engineering. In many cases of practical interest, they canbe solved by means of a well-known methodology known as the AkaikeInformation Criterion [2]. The original work of Akaike has been extendedby a number of authors in order to handle multidimensional signals, butalways under the assumption that the signal has a known dimension. TheST IRC methodology developed in [1] has the particular feature that thedimension of the received signal is a model parameter. The originalmultidimensional signal is stacked to form a new, artificial, receivedsignal of even higher dimension. A derivation and explanation as to whyAkaike [2] is not applicable in a straight forward manner to signals ofpossibly varying dimension is shown in Appendix A for the interestedreader. The present invention therefore consists of a mathematicalexpression to estimate the model order for indirect ST IRC [1]. It hasbeen derived though a non-trivial extension of the methodology of Akaiketo suit the indirect ST IRC framework. The interested reader is referredto Appendix B for the complete derivation. The main points of the methodwill be described below.

Embodiments of the present invention will be described with reference toFIG. 2 and FIG. 3.

Basically, with reference to FIG. 2, the present invention comprisesgenerating S0 a spatially and temporally stacked signal model bystacking successive samples of temporally adjacent received signalvectors and corresponding training vectors. Subsequently, a noisevariance matrix representative of each hypothesized synchronizationposition, channel length and stacking order is computed S1 based on thegenerated stacked training symbols. Finally, a best synchronizationposition for the received signal is determined S2 based on the stackedtraining vectors by jointly determining the best synchronizationposition for the received signal and estimating a channel length and astacking order for said signal model based on the stacked trainingvectors.

Subsequently, with reference to FIG. 3, the thus jointly determinedsynchronization position, channel length estimate, and stacking orderfor the model signal are utilized in subsequent steps of computing anoise covariance estimate S3, computing an estimated of a fictitiouschannel S4, and computing an estimate of the whitened channel S5 in aknown manner.

To further understand the framework of the present invention, an indepth discussion of the mathematics and the derivation of theexpressions used is included below.

The Akaike Information Criterion and ST IRC

We begin by stating a version of the Akaike Information Criterion [2]which is the starting point for the present invention. Let Y denote thematrix of received, complex-valued data samples of dimensionN_(branches)×N_(samples). N_(branches) can be thought of as a spatialdimension, whereas N_(samples) is the temporal dimension. For example,N_(branches) could be the number of branches in an antenna array, whileN_(samples) could be the number of samples received( Θ|_(q)−{circumflex over (Θ)})*I( Θ|_(q) ,Y)( Θ|_(q)−{circumflex over(Θ)})  (2)is asymptotically distributed as χ_(2q) ². (Denote the Chi-squared withn degrees of freedom by χ_(n) ².) The factor 2 in front of the q stemsfrom the fact that Θ is a vector of complex quantities. Letdim({circumflex over (Θ)})=2q.

The model selection methodology can be used successfully when differentchoices of the unknown parameters Θ yield different numbers of samplesN_(samples). i.e. when N_(samples)=N_(samples)(Θ).

The ST IRC methodology is rather special since there is a dependencebetween the spatial dimension and the parameter choice. In other words,N_(branches)=N_(branches)(Θ). However, in this case, Equation (1) doesnot always result in reasonable parameter choices. The reason for thiswill be explained in more detail later on. Thus, a straightforwardapplication of the Akaike methodology will not solve the model selectionproblem for ST IRC.

For the further description, it is necessary with a few preliminarydefinitions from the disclosure in [1].

Consider the following typical, dispersive multiple-input-multipleoutput (MIMO) signal model with additive noise:

$\begin{matrix}{{{r\left\lbrack {n + n_{0}} \right\rbrack} = {{\sum\limits_{k = 0}^{L - 1}{{C\lbrack k\rbrack}{s\left\lbrack {n - k} \right\rbrack}}} + {v\lbrack n\rbrack}}},} & (3)\end{matrix}$for n=L−1, L, Λ, N−1, where N denotes the length of the trainingsequence (N=26 for GSM/EDGE), r[n] denotes a N_(r)-dimensional received(column) vector, n₀ denotes the synchronization position, which is thetime index of in each branch. Assume that p is the pdf (probabilitydistribution function) of Y and that it belongs to a known familyparameterized by Θ.

An estimate {circumflex over (Θ)} of Θ may be chosen according to thefollowing equation.

$\begin{matrix}{\hat{\Theta} = {\arg{\min\limits_{\Theta}{\frac{1}{N_{samples}}\left( {{{- \log}\;{p\left( Y \middle| \Theta \right)}} + {\dim(\Theta)}} \right)}}}} & (1)\end{matrix}$where dim(Θ) is the number of degrees of freedom associated with theparticular parameter vector Θ.

It will be illuminating to review some aspects of the derivation ofEquation (1). The following assumptions will be valid throughout thedescription. Assume that Y=└Y₁, . . . , Y_(N) _(samples) ┘, where eachentry Y_(k) is a column vector of dimension N_(branches). Assume alsothat the random vectors (Y_(k)) are i.i.d. and that Y_(k)≈N(μ(k, Θ), Q),where Q is an unknown positive definite matrix. Moreover, assume thatfor each k the function μ(k.) is a linear function.

The main ingredients in the derivation of Equation (I), starting fromthe Kullback-Leibler divergence, are the law of large numbers and theasymptotic consistency and normality of the maximum likelihood estimatorfor vector valued random variables. These are guaranteed by thepreviously stated assumptions. (The Gaussian pdf and the lineardependence of the mean upon the unknown parameter vector ensure that thesmoothness requirements are fulfilled). Denote the true parameter vectorby Θ.

Akaike discovered [2] that the log-likelihood is a biased estimator ofthe Kullback-Leibler divergence. He also showed that the bias can beapproximated by the dimension of {circumflex over (Θ)}. Let I( Θ, Y) bethe Fisher information matrix. Assume that {circumflex over (Θ)}consists of q complex parameters. The key estimate is that the quadraticform the first symbol of the training period, {C[k]} denotes a N, by N,MIMO channel response of length L, {s[n]} denotes possiblyN_(t)-dimensional training vector, and {v[n]} denotes a spatially andtemporally colored noise process used to model the interference. Also,let ┌ denote a (finite) set of possible sync positions.

A stacked vector notation will be employed in the following description.Let r^(M−1)[n] a vec([r[n], r[n−1], Λ r[n−M]]) be a vector formed bystacking {r[k]}_(k=n−M) ^(n) in columns, where M denotes the model orderor “stacking order”, and for any matrix A, vec(A) is the vector formedby stacking columns of A one by one into a single vector, i.e. usingtypical Matlab notation, vec(A) a A(:). Similarly, letv^(M+1)[n]≡vec([v[n],v[n−1,], Λ v[n−M]]) denote the correspondingstacked noise vector, and s^(L)[n]=vec([s[n],s[n−1,],Λ,s [n−L+1]])denote the corresponding stacked training vector.

Rewriting the signal model in Equation (3) by stacking (M+1) temporallyadjacent received vectors, provides the following stacked signal model:

$\begin{matrix}{\mspace{79mu}{{{r^{M - 1}\left\lbrack {n + n_{0}} \right\rbrack} = {{{T(C)}{s^{L + M}\lbrack n\rbrack}} + {v^{M + 1}\lbrack n\rbrack}}},\mspace{79mu}{where}}} & (4) \\{{T(C)} \equiv \begin{bmatrix}{C\lbrack 0\rbrack} & {C\lbrack 1\rbrack} & \Lambda & {C\left\lbrack {L - 1} \right\rbrack} & 0 & \Lambda & 0 \\0 & {C\lbrack 0\rbrack} & {C\lbrack 1\rbrack} & \Lambda & {C\left\lbrack {L - 1} \right\rbrack} & 0 & M \\M & M & M & M & M & M & M \\M & 0 & {C\lbrack 0\rbrack} & {C\lbrack 1\rbrack} & \Lambda & {C\left\lbrack {L - 1} \right\rbrack} & 0 \\0 & \Lambda & 0 & {C\lbrack 0\rbrack} & {C\lbrack 1\rbrack} & \Lambda & {C\left\lbrack {L - 1} \right\rbrack}\end{bmatrix}} & (5)\end{matrix}$

is an (M+1)×(L+M) block Toeplitz matrix of block size N_(r)×N_(t). A keymodel assumption is that the (expanded) noise vector process{v^(M+1)[n]} is independent and identically distributed (IID), and letΛ≡E[v^(M+1)[n](v^(M+1)[n])^(H)] be the covariance matrix of v^(M+1)[n].

According to [1], given L, M, n₀, the covariance matrix {circumflex over(Λ)} can be estimated by

$\begin{matrix}{{\hat{\Lambda}\left( {L,M,n_{0}} \right)} = {{\sum\limits_{n = {L + M - 1}}^{N - 1}{{r^{M + 1}\left\lbrack {n + n_{0}} \right\rbrack}\left( {r^{M + 1}\left\lbrack {n + n_{0}} \right\rbrack} \right)^{H}}} - {\quad{{\left\lbrack {\sum\limits_{n = {L + M - 1}}^{N - 1}{{r^{M + 1}\left\lbrack {n + n_{0}} \right\rbrack}\left( {s^{L + M}\lbrack n\rbrack} \right)^{H}}} \right\rbrack\left\lbrack {\sum\limits_{n = {L + M - 1}}^{N - 1}{{s^{L + M}\lbrack n\rbrack}\left( {s^{L + M}\lbrack n\rbrack} \right)^{H}}} \right\rbrack}^{- 1}{\quad{\left\lbrack {\sum\limits_{n = {L + M - 1}}^{N - 1}{{s^{L + M}\lbrack n\rbrack}\left( {r^{M + 1}\left\lbrack {n + n_{0}} \right\rbrack} \right)^{H}}} \right\rbrack.}}}}}} & (6)\end{matrix}$

The model order selection problem for Indirect ST IRC [1] solved by thepresent invention consists of estimators {circumflex over (L)},{circumflex over (M)} for L, M. As usual, the symbol ^ on top of aquantity denotes an estimator of said quantity.

Without loss of generality assume that the stacking order M is limitedto the values M₀=0<M₁<Λ<M_(m)=M_(max) and that M_(max)+1 is divisible byM_(k)+1 for all 0<k<m. (Given any desired set of stacking ordersM₀=0<M₁<Λ<M_(n), it is always possible to define

${M_{n + 1} = {M_{\max} = {{\prod\limits_{p = 1}^{n}\;\left( {M_{p} + 1} \right)} - 1}}},$and the hypothesis is fulfilled.) Assume also that the channel length Lis one of the positive integers L₀<L₁<Λ<L_(p).

Next, define the integers

$\begin{matrix}{{{\gamma(M)} = \frac{M_{\max} + 1}{M + 1}},{{{for}\mspace{14mu} M} = M_{k}},{0 \leq k \leq {m.}}} & (7)\end{matrix}$

The channel length and the stacking order are chosen according to thefollowing expression, which also gives the synchronization position.

$\begin{matrix}{{\left\lbrack {\hat{L},\hat{M},{\hat{n}}_{0}} \right\rbrack = {\underset{\underset{n_{0} \in \Pi}{M = {M_{k}:{0 \leq k \leq m}}}}{\underset{L = {L_{q}:{0 \leq q \leq p}}}{\arg\;\min}}\left\{ {{{\hat{\Lambda}\left( {L,M,n_{0}} \right)}}^{\gamma{(M)}}\left( {\exp\left( \frac{p\left( {L,M} \right)}{N - L + 1 - M} \right)}^{{({M_{\max} + 1})}N_{r}} \right)} \right\}}},} & (8)\end{matrix}$where p(•,•) is a cost function and the parallel bars ∥ denote thedeterminant of a matrix. The use of the methodology of Akaike [2] yieldsthe cost functionp(L,M)=2L+2M.  (9)

In order to obtain more flexibility in the model choice it is possibleto change the cost function. For example p(L,M)=C₁L+C₂M, where C₁, C₂are constants. Other choices of the penalty function are possible, basedon simulations or heuristics.

Note that the expression in brackets in Equation (8) can be replaced byany monotone increasing function or mapping of it, and the same resultswill be obtained. This could be advantageous for numerical reasons. Forexample. Equation (8) is equivalent to the expression below:

$\begin{matrix}{\left\lbrack {\hat{L},\hat{M},{\hat{n}}_{0}} \right\rbrack = {\underset{\underset{n_{0} \in \Pi}{M = {M_{k}:{0 \leq k \leq m}}}}{\underset{L = {L_{q}:{0 \leq q \leq p}}}{\arg\;\min}}{\left\{ {{{\hat{\Lambda}\left( {L,M,n_{0}} \right)}}^{\frac{1}{{({M + 1})}N_{r}}}{\exp\left( \frac{p\left( {L,M} \right)}{N - L + 1 - M} \right)}} \right\}.}}} & (10)\end{matrix}$

Note that Equation (10) is independent of M_(max). The completederivation of Equation (8) is given in Appendix B attached to thisdisclosure.

To further support and illustrate the impact of the present invention, aseries of simulations have been performed.

FIG. 4 illustrates the relation between Block Error Rate (BLER) andSignal to Noise ratio (SNR) for various models in a sensitivity limitedenvironment and how the present invention enables an intelligent choiceof the best model for different environmental conditions. The differentlines represent different combinations of channel length and stackingorder. From the diagram, it is obvious that the different models behavedifferently with different SNR. By utilizing the invention (withvariable stacking order) it is possible to always attain the best modelchoice (as indicated by the solid line). If a fixed model had been used,the result would be differently (as illustrated by the various dottedlines).

Similarly, FIG. 5 illustrates how BLER varies with Carrier toInterference ratio (C/I) for an interference dense environment.

An arrangement according to the invention will be described withreference to FIG. 6. The arrangement is configured for receiving signalsover one or multiple antennas (as indicated by the dotted lines) througha general input/output unit I/O. Further, the arrangement comprises aunit 10 for generating a stacked signal model by stacking successivesamples of spatially and temporally adjacent received signal vectors andcorresponding training vectors. The samples typically comprise inputfrom different antennae. In addition, the arrangement comprises acomputational unit 20 for computing a noise covariance matrix for eachcombination of hypothesized synchronization position, channel length andstacking order based on the previously provided stacked trainingsymbols. The computational unit 20 is further adapted to compute thecovariance matrixes in such a manner as to neglect the Toeplitzstructure of the channel. Finally, the arrangement according to theinvention comprises a unit 30 for determining a best synchronizationposition for the received signal, based on the stacked training vectors.The determining unit 30 is configured for jointly determining thesynchronization position, a channel length estimate, and a stackingorder of the signal model. Thus, the arrangement is configured forproviding a “triplet” of model parameters.

The methods and arrangement according to the invention can beimplemented in the receivers of base transceiver stations or in themobile stations, or in some other part of the system where a receiveralgorithm is applied. Further, the MS or BTS may be provided with one ormore antennas.

The known method of joint whitening/synchronization disclosed in [1]have great potential for example in receiver algorithms for upcomingGSM/EDGE dual antenna terminals or in the BTS receiver algorithms forEvolved EDGE. However, the order of the model is unknown in practicalapplications in wireless communications. The present invention providesa statistic for the model order selection problem based on acceptedstatistical methodology. The statistic has low computational complexityand is easy to implement in digital signal processors or otherelectronic circuitry.

It will be understood by those skilled in the art that variousmodifications and changes may be made to the present invention withoutdeparture from the scope thereof, which is defined by the appendedclaims.

REFERENCES

-   1. Patent application WO2006/136875 “Method of Joint Synchronization    and Noise Covariance Estimation based on a Spatially and Temporally    Stacked Signal Model”.-   [2] Akaike, Hirotugu (1974). “A new look at the statistical model    identification”. IEEE Transactions on Automatic Control 19 (6):    716-723.

APPENDIX A

Fixed Stacking Order

Consider a fixed stacking order M and derive a statistic to choose thelength of the channel impulse response, e.g. channel length.

The model selection problem for ST IRC is about finding a suitableestimate of H which is the (fictious) channel impulse response. To bespecific, assume that the stacking order is M≧0, the channel has L+1taps, there are N_(tr) training symbols s, the synchronization positionis n₀, and the received signal y has N_(r) branches. Writing

$\begin{matrix}{{{y^{M}(n)} = \begin{bmatrix}{y\left( {1,{n + n_{0}}} \right)} \\M \\{y\left( {N_{r},{n + n_{0}}} \right)} \\M \\{y\left( {1,{n + n_{0} - M}} \right)} \\M \\{y\left( {N_{r},{n + n_{0} - M}} \right)}\end{bmatrix}},} & ({A1}) \\{{{v^{M}(n)} = \begin{bmatrix}{v\left( {1,n} \right)} \\M \\{v\left( {{\left( {M + 1} \right)N_{r}},n} \right)}\end{bmatrix}},{and}} & \left( {A\; 2} \right) \\{{{s^{M}(n)} = \begin{bmatrix}{s(n)} \\M \\{s\left( {n - L - M} \right)}\end{bmatrix}},} & \left( {A\; 3} \right)\end{matrix}$the ST IRC signal model is the following

$\begin{matrix}{{{y^{M}(n)} = {{\begin{bmatrix}{H\left( {1,0} \right)} & K & {H\left( {1,{L + M}} \right)} \\M & \; & M \\{H\left( {{\left( {M + 1} \right)N_{r}},0} \right)} & \Lambda & {H\left( {{\left( {M + 1} \right)N_{r}},{L + M}} \right)}\end{bmatrix}{s^{M}(n)}} + {v^{M}(n)}}},\mspace{79mu}{n = {L + M}},\Lambda,{N_{tr} - 1}} & ({A4})\end{matrix}$where the residuals ν^(M)(n) are i.i.d. and have anN_(r)(M+1)×N_(r)(M+1) covariance matrix ΛQ _(M) =E[ν ^(M)(n)ν^(M)(n)*]  (A5)

This model can also be written in matrix formY=HS+V  (A6)

As usual S denotes a suitable Toeplitz matrix of training symbols and His the fictious stacked channel matrix. The dimension of Y isN_(r)(M+1)×(N_(tr)−L−M). The dimension of H is (M+1)N_(r)×(L+1+M). Thematrix V has the same dimension as Y. The columns of V are complexGaussian, i.i.d., zero mean with (unknown) covariance Λ_(M).

The assumptions imply that the columns of Y are i.i.d. complex Gaussianwith covariance Λ_(M) and non-zero mean. The mean is given by thecolumns of HS. Thereforep(Y|H,Λ _(M))=(π^(−N) ^(r) ^((M+1))|Λ_(M)|⁻¹exp(−tr({circumflex over(Λ)}_(M)Λ_(M) ⁻¹)))^(N) ^(samples) ,  (A7)whereN _(samples) =N ^(tr) −L−M  (A8)and

$\begin{matrix}{{{\hat{\Lambda}}_{M} = {\frac{1}{N_{samples}}\left( {Y - {HS}} \right)\left( {Y - {HS}} \right)^{*}}},} & ({A9})\end{matrix}$

Note that {circumflex over (Λ)}_(M) is dependent on H. We can now writethe log-likelihood−log p(Y|H,Λ _(M))=[N _(r)(M−1)log π−log|Λ_(M) |−tr({circumflex over(Λ)}_(M)Λ_(M) ⁻¹)]N _(samples)  (A10)

For any given H the expression on the left hand side of (A10) isminimized by taking Λ_(M)={circumflex over (Λ)}_(M). Moreover, using theindirect ST IRC algorithm [1] we find an estimate Ĥ^(D) of H thatmaximizes the log-likelihood, or equivalently −log|Λ_(M)|. Let's call{circumflex over (Λ)}_(M) ^(D) the sample covariance matrix associatedwith Ĥ^(D). We then compute

$\begin{matrix}{{\min\limits_{H \cdot Q_{M}}\left\{ {{- \log}\;{p\left( Y \middle| {H \cdot \Lambda_{M}} \right)}} \right\}} = {\left\lfloor {{{N_{r}\left( {M + 1} \right)}\log\;\pi} + {\log{{\hat{\Lambda}}_{M}^{D}}} + {{tr}\left( I_{N_{r}{({M + 1})}} \right)}} \right\rbrack{\quad{N_{samples} = {\quad{\left\lbrack {{{N_{r}\left( {M + 1} \right)}\log\;\pi} + {\log{{\hat{\Lambda}}_{M}^{D}}} + N_{r{({M + 1})}}} \right\rbrack N_{samples}}}}}}} & ({A11})\end{matrix}$

Using Equation (1) and Equation (A11) and eliminating the termsindependent of L we arrive at the expression

$\begin{matrix}{\hat{L} = {\arg\;{\min\limits_{L}\left( {{\log{{\hat{\Lambda}}_{M}^{D}}} + \frac{\dim\left( {\hat{H}}^{D} \right)}{N_{samples}}} \right)}}} & ({A12})\end{matrix}$

In order to avoid the calculation of logarithms we take exponential inEquation (A12).

$\begin{matrix}{\hat{L} = {\arg\;{\min\limits_{L}{{{\hat{\Lambda}}_{M}^{D}}{\mathbb{e}}^{\frac{\dim{({\hat{H}}^{D})}}{N_{samples}}}}}}} & ({A13})\end{matrix}$

Next we calculate dim(Ĥ^(D)). This is straightforward, with the warningthat one complex parameter is counted as 2 (real parameters). We obtaindim(Ĥ^(D))=2(M+1)N_(r)(L+1+M). This gives

$\begin{matrix}{\hat{L} = {\arg\;{\min\limits_{L}{{{\hat{Q}}_{M}^{D}}{\mathbb{e}}^{\frac{2{({M + 1})}{N_{r}{({L + 1 + M})}}}{N_{samples}}}}}}} & ({A14})\end{matrix}$Defining the Quantity

$\begin{matrix}{{{AIC}(L)} = \left( {\mathbb{e}}^{\frac{2{({L + 1 + M})}}{N_{tr} - L - M}} \right)^{{({M + 1})}N_{r}}} & ({A15})\end{matrix}$we obtain

$\begin{matrix}{\hat{L} = {\arg\;{\min\limits_{L}{{{\hat{Q}}_{M}^{D}} \cdot {{AIC}(L)}}}}} & ({A16})\end{matrix}$

APPENDIX B

Variable Stacking Order

Although the mathematical calculations of the previous subsection can becarried out for variable stacking order M, the result is not usefulbecause the methodology breaks down. To see why, let's look at Equations(1) and (A11). As usual, given any positive integer p, we will callI_(p) the identity matrix of dimension p×p. In sensitivity scenarios{circumflex over (Λ)}_(M) ^(D)≈σ²I_(N) _(r) _((M+1)), where σ² is thenoise power. Therefore |{circumflex over (Λ)}_(M) ^(D)|≈σ^(2N) ^(r(M+1)). Thus, as σ²→0 determinant of the matrix with the largest stackingorder M will dominate all other terms in the minimization in Equation(1). Consequently, for sufficiently small σ² the highest stacking orderwill always be chosen, even in sensitivity cases.

This difficulty can be solved as follows. Let us say that only thestacking orders M₀=0<M₁<Λ<M_(m)=M_(max) are allowed. Without loss ofgenerality we assume that M_(m)+1 is divisible by M_(k)+1 for all 0<k<m.(Given any desired set of stacking orders M₀=0, . . . , M_(k) we canalways add M_(k+1)=M_(max)=┌_(p=1) ^(k)(M_(p)+1)−1). The stacked modelof order M_(k) can be embedded in the stacked model of order M_(max).For convenience, define the integers γ_(k)

$\begin{matrix}{{\gamma_{k} = \frac{M_{\max} + 1}{M_{k} + 1}},{k = 0},\Lambda,m} & ({B1})\end{matrix}$and the fictious or hypothesized stacked channel matrices H_(k)

$\begin{matrix}{H_{k} = \begin{bmatrix}{H\left( {1,0} \right)} & \Lambda & {H\left( {1,{L + M_{k}}} \right)} & 0_{1 \times {({M_{\max} - M_{k}})}} \\M & \; & \; & M \\0_{1 \times {({M_{\max} - M_{k}})}} & {H\left( {{\left( {M_{\max} + 1} \right)N_{r}},0} \right)} & \Lambda & {H\left( {{\left( {M_{\max} + 1} \right)N_{r}},{L + M_{k}}} \right)}\end{bmatrix}} & ({B2})\end{matrix}$where k=0, . . . , m of dimension (M_(max)+1)N_(r)×(L+1+M_(max)). Theexpression 0_(1×(M) _(max) _(−M) _(k) ₎ denotes a matrix of zeros indimension 1×(M_(max)−M_(k)).

Using the notation introduced in the previous section, the stacked modelfor order M_(k) embedded in the higher dimension M_(max) can be writtenin the formy _(k) ^(M) ^(max) (n)=H _(k) s ^(M) ^(max) (n)+v ^(M) ^(max) (n)  (B3)with the assumption that the covariance matrixΛ_(k) =E└v ^(M) ^(max) (n)·v ^(M) ^(max) (n)*┘=I _(γk)

Λ_(M) _(k)   (B4)Here

is the Kronecker product and Λ_(M) _(k) is the(M_(k)+1)N_(r)×(M_(k)+1)N, covariance matrix of the stacked model oforder M_(k).

Notice that this embedded version is not identical to the originalformulation and that we have not imposed any structural constraints onH_(k). In fact. H_(k) is composed of γ_(k) (possibly) differentsub-matrices, all of which are convergent to the channel of stackingorder M_(k) as N_(samples)→∞. Because of this we say that the twoformulations are asymptotically equivalent. The difference between theoriginal and the embedded formulations is due to border effects, sincethe stacking reduces the number of used samples in the temporaldimension.

Now, we can apply the methodology developed in the previous subsectionto the embedded models, since all the model candidates have the samespatial dimension (M_(max)+1)N_(r). Exactly the same argument leading toEquation (A13) yields

$\begin{matrix}{\left\lbrack {L,M_{k}} \right\rbrack = {\arg\;{\min\limits_{L,k}{{{\hat{\Lambda}}_{k}^{D}}{\exp\left( \frac{\dim\left( {\hat{H}}_{k}^{D} \right)}{N_{samples}} \right)}}}}} & ({B5})\end{matrix}$

Recall that {circumflex over (Λ)}_(k) ^(D) is block diagonal. Hence|{circumflex over (Λ)}_(k) ^(D)|=|{circumflex over (Λ)}_(M) _(k)^(D)|^(γk)  (B6)Moreover,dim(Ĥ _(k) ^(D))=2(M _(max)+1)N _(r))(L+1+M _(k))  (B7)

Hence Equation (B5) can be rewritten as

$\begin{matrix}{\left\lbrack {\hat{L},{\hat{M}}_{k}} \right\rbrack = {\arg\;{\min\limits_{L,k}{{{\hat{\Lambda}}_{M_{k}}^{D}}^{\gamma_{k}}{\mathbb{e}}^{\frac{2{N_{r}{({M_{\max} + 1})}}{({L + 1 + M_{k}})}}{N_{samples}}}}}}} & ({B8})\end{matrix}$

Finally, defining the factor

$\begin{matrix}{{{AIC}\left( {L,M_{k}} \right)} = \left( {\mathbb{e}}^{\frac{2{({L + 1 + M_{k}})}}{N_{tr} - L - M_{k}}} \right)^{{({M_{\max} + 1})}N_{r}}} & ({B9})\end{matrix}$we arrive at the statistic

$\begin{matrix}{\left\lbrack {\hat{L},{\hat{M}}_{k}} \right\rbrack = {\arg\;{\min\limits_{L,k}{{{\hat{\Lambda}}_{M_{k}}^{D}}^{\gamma_{k}}{{AIC}\left( {L,M_{k}} \right)}}}}} & ({B10})\end{matrix}$

In practice it is desirable to have some flexibility in the choice ofthe channel lengths or stacking orders, in order to favor/punish someparticular parameter sets. Thus Equation (B9) can be modified asfollows.

$\begin{matrix}{{{AIC}\left( {L,M_{k}} \right)} = \left( {\mathbb{e}}^{\frac{{penalty}{({L,M_{k}})}}{N_{tr} - L - M_{k}}} \right)^{{({M_{\max} + 1})}N_{r}}} & ({B11})\end{matrix}$where penalty(•,•) is some cost function. A simple choice in is thelinear functionpenalty(L,M)=L _(AIC)·(L+1)+M _(AIC) ·M  (B12)for suitable constants L_(AIC) and M_(AIC). The values L_(AIC)=2.M_(AIC)=2 give back Equation (B9). These constants can also be tunedthrough simulations.

Observe that the term |{circumflex over (Λ)}_(M) _(k) ^(D)|^(γk)AIC(L,M_(k)) on the right hand side of Equation (B11) may be replacedwith ƒ(|{circumflex over (Λ)}_(M) _(k) ^(D)|^(γk) AIC(L,M_(k))) for anymonotonically increasing function ƒ, without altering the model choice.This can be useful when performing the calculations in the target. Forexample, using ƒ(x)=x^(α) with

${\alpha = \frac{1}{\left( {M_{\max} + 1} \right)N_{r}}},$it can be seen that Equation (B11) is equivalent to

$\begin{matrix}{\left\lbrack {\hat{L},{\hat{M}}_{k}} \right\rbrack = {\arg\;{\min\limits_{L,k}{{{\hat{\Lambda}}_{M_{k}}^{D}}^{\frac{1}{{({M_{k} + 1})}N_{r}}}\left( {\mathbb{e}}^{\frac{{penalty}{({L,M_{k}})}}{N_{tr} - L - M_{k}}} \right)}}}} & ({B13})\end{matrix}$

Note that Equation (B13) is independent of M_(max).

Since Equation (1) is itself based on an asymptotic approximation, amodel selection criterion for the embedded models should also be validfor the original (i.e. not embedded) stacked models.

1. A method of enabling model order selection for joint channelsynchronization and noise covariance estimation of at least one receivedsignal in a wireless communication network, said method comprising:generating a spatially and temporally stacked signal model by stackingsuccessive samples of temporally adjacent received signal vectors andcorresponding training vectors; computing a noise variance matrix foreach hypothesized synchronization position, channel length and stackingorder, based on the stacked training symbols; and determining a bestsynchronization position for the received signal, based on the stackedtraining vectors; wherein said determining comprises jointly determiningthe best synchronization position for the received signal and estimatinga channel length and a stacking order for said signal model based on thestacked training vectors.
 2. The method of claim 1, wherein saiddetermining comprises: defining a model parameter dependent on astacking order of said generated stacked signal model; and calculatingan estimate of the channel length, the stacking order and the bestsynchronization position based on a minimum of a function dependent ofthe determinant of said noise covariance matrix, said model parameterand a penalty function.
 3. The method of claim 2, wherein saiddetermining further comprises: calculating the estimate of the channellength, the stacking order, and the best synchronization position basedon a minimum of a function dependent of the determinant of said noisecovariance matrix to the power of said model parameter and apredetermined exponential function dependent on a penalty function. 4.The method of claim 3, further comprising calculating said estimate ofthe channel length, the stacking order and the best synchronizationposition based on a minimum of any monotonically increasing mapping ofsaid function.
 5. The method of claim 3, wherein said penalty functionp(L,M) is a positive function of L and M, where L is the channel length,and M is the stacking order.
 6. The method of claim 5, wherein saidpenalty function p(L,M) is represented by the expressionp(L,M)=CL+DM, where C, D are positive constants, L is the channellength, and M is the stacking order.
 7. The method of claim 6, whereinC=2 and D=2.
 8. The method of claim 2, wherein said model parameter γ(M)is represented by the expressionγ(M)=1/((M+1)N _(r)) where M is the stacking order, and N_(r) is thenumber of branches in the received signal.
 9. A receiver arrangementconfigured for enabling model order selection for joint channelsynchronization and noise covariance estimation of at least one receivedsignal in a wireless communication network, said receiver arrangementcomprising: a stacking circuit configured for generating a spatially andtemporally stacked signal model by stacking successive samples oftemporally adjacent received signal vectors and corresponding trainingvectors; a noise variance estimation circuit configured for computing anoise variance matrices for all hypothesized synchronization positions,channel lengths and stacking orders, based on the stacked trainingsymbols; a synchronization determining circuit configured fordetermining a best synchronization position for the received signal,based on the stacked training vectors; and wherein said synchronizationdetermining circuit is configured for jointly determining the bestsynchronization position for the received signal and estimating achannel length and a stacking order for said signal model based on thestacked training vectors.
 10. The receiver arrangement of claim 9,wherein said receiver arrangement comprises a mobile station receiverarrangement implemented in a mobile station configured for operation inthe wireless communication network.
 11. The receiver arrangement ofclaim 10, wherein said receiver arrangement comprises a base stationreceiver arrangement implemented in a base station configured foroperation in the wireless communication network.